Field
The present disclosure relates to tire treads and more particularly to treads comprising grooves comprising flexible devices that form devices that reduce the noise generated by air going into resonance inside these grooves during driving.
Description of Related Art
It is known that as a tire that is running comes into contact with a road surface air is made to circulate in a sort of pipe formed by part of a groove, notably, although not exclusively, a groove of circumferential overall orientation, and the road surface itself, this pipe being open at both ends.
In this pipe there is formed, in the contact patch, a column of air which goes into vibration and the resonant frequency of which depends on the length between the two ends of the pipe and, therefore, on the length of groove in contact with the road surface.
This resonance of the air in the grooves has the result of generating, in a vehicle fitted with these tires, a noise that can be heard inside the vehicle and outside the vehicle.
These inside and outside noises usually correspond to a frequency of 1 kHz or thereabouts, which is a frequency to which the human ear is particularly sensitive. In order to reduce such resonant noise, it is known practice to arrange, in each circumferentially oriented or generally circumferentially oriented groove, a plurality of relatively thin closure membranes or blades made of a rubbery material, each closure membrane or blade occupying the entire cross section of the groove or, at least, a large proportion of this cross section. Each closure membrane may extend from the bottom of the groove or may be fixed to at least one of the walls delimiting the said groove.
Thanks to these flexible closure membranes, the length of the column of air in each circumferential groove is reduced by comparison with the total length of the groove in the contact patch, and this leads to a change in the resonant frequency. This change in frequency is towards resonant frequency values to which the human ear is less sensitive. The number of flexible membranes to form a running-noise-reducing device needs to be determined so that the length of groove that can go into resonance in contact with the road surface are suited to delivering resonances that are less perceptible to the ear.
What is meant by relatively thin is that each flexible closure blade or membrane is able to flex only under suitable conditions achieved when running on a water-covered road surface, notably in rainy weather. Specifically, in order to maintain a function of draining away water between the road surface and the tire, it is necessary for this membrane to flex under the action of the water pressure in order to open the cross section of the groove to the flow of the water.
Several solutions of this type for reducing the resonance of the column of air vibrating in the grooves have been proposed, for example in patent document FR2715891.
In the case of noise-reducing devices formed of at least one flexible blade, each flexible blade being connected to the bottom of a groove, the device is seen to be highly effective in the new condition. However, this effectiveness diminishes after the tread comprising grooves having such devices becomes partially worn. One reason for this is the increase in flexural rigidity of each blade when it has become partially worn.
It is an object of embodiments of the invention to improve the effectiveness of such noise-reducing devices with tread wear.
Definitions:
The tread surface of a tread corresponds to the surface of the tread that comes into contact with the ground when a tire fitted with such a tread is being driven on.
A groove corresponds to a space formed in a tread, this space being delimited by walls of material of raised elements (blocks, ribs), these walls being joined together by a groove bottom distant from the tread surface by a maximum distance equal to the depth of the groove. The groove bottom corresponds to that part of the groove that is situated radially on the inside of a legal tread wear limit beyond which the tread no longer meets the legal requirements for use. In general, a plurality of tire tread wear limit indicators are positioned in one or more grooves. The radially external part of these indicators corresponds to the transition between the walls of a groove and the bottom of the said groove.
A block is a raised element formed on a tread, this element being delimited by voids or grooves and comprising lateral walls and a contact face, the latter—which is intended to come into contact with the road surface during driving—forming part of the tread surface of the tread.
A rib is a raised element formed on a tread, this element being delimited by two grooves. A rib comprises two lateral walls and a contact face, the latter being intended to come into contact with the road surface.
A radial direction means a direction perpendicular to the axis of rotation of the tire (this direction corresponds to the direction of the thickness of the tread).
An axial direction means a direction parallel to the axis of rotation of the tire.
A circumferential direction means a direction which is tangential to any circle centred on the axis of rotation. This direction is perpendicular both to the axial direction and to a radial direction.